1. Field of the Invention
The present invention relates to a roll of the type used for smoothing paper webs. The roll has a hard roll core which can be a metal and an outside surface utilizing a resilient covering layer. The covering layer may be a resilient matrix material with fibers embedded in the matrix material. Furthermore, the invention is directed to a process for producing such a roll.
2. Discussion of Background Information
Resilient rolls of this type are typically used, for example, in the calendering of paper webs. Such calenders often use an elastic roll together with a hard roll in forming a press nip. The paper web is calendered by feeding it through one or more of these nips. The hard rolls generally have a very smooth surface and are made of, for example, steel or hard cast iron. They function in smoothing that side of the paper web which faces it. Resilient rolls which act on the opposite side of the paper web have the effect of evening and compacting the paper web in the press nip. The resilience of this second or opposite roll in the nip acts to limit intensive compaction of the paper web, which would lead to a specky appearance of the paper web. Such rolls are generally large and typically have lengths of from about 6 to 12 meters and diameters from about 800 to 1500 mm. Moreover, they are designed to withstand line forces up to approximately 600 N/mm and compressive stresses up to about 50 N/mm.sup.2.
The tendency in paper manufacture is for calendering to be carried out on-line, that is to say the paper web leaving the papermaking machine or coating machine is led immediately through the paper smoothing device (calender). This design places high requirements or demands on the rolls of the calender or smoothing device. In particular, this design subjects the rolls to higher temperatures so that they are require to have temperature resistance. The high transport speeds of the paper web, necessitated by on-line operation, and the associated high rotational speeds of the calender rolls increase the alternating flexure frequency of the rolls. It is these factors which in turn leads to increased roll temperatures.
These high temperatures which are produced in on-line operation lead to problems which, in the case of conventional resilient rolls, can lead to the destruction of the synthetic covering. Such conventional synthetic coverings can function only with a maximum temperature differences of about 20.degree. C. over the width of the roll. Moreover, the polymers normally used for the roll coating have a significantly higher coefficient of thermal expansion than the steel rolls or hard cast rolls normally used. Thus, when there is an increase in the temperature of the rolls, high axial stresses occur between the steel roll or hard cast roll and the synthetic coating which is connected to it.
Moreover, such rolls also experience high stresses in localized regions of the roll due to these regions being heated more so than surrounding areas. Such hot spots in the synthetic coating can cause the synthetic layer to separate or burst from the metal roll.
These hot spots can occur when, in addition to the mechanical stresses and the relatively high temperatures experienced by the rolls, there are crystallization points in the form, for example, of faulty adhesive bonds between the layer and the metal. Additionally, deposits or above-average bulges in the resilient covering which result from creases or foreign bodies on the paper web can produce these hot spots or crystallization points. In these cases, the temperature of these crystallization points often rises from normally about 80.degree. C. to 90.degree. C. to more than 150.degree. C., which results in the aforementioned destruction of the synthetic layer.